Styrene is one of the most important monomers in the modern petrochemical industry. It is used as a raw material in the production of many plastics, in particular polystyrene, as well as rubbers and resins. In 2006, United States consumption of styrene was about 14.4 billion pounds.
The most common method of production of styrene monomer (SM) is by dehydrogenation of ethylbenzene (EB). One process for production of styrene monomer from EB is by direct dehydrogenation. In this process, excess superheated steam near 800° C. is combined with EB in a low-pressure adiabatic reactor containing a potassium-promoted iron oxide catalyst. The reaction temperature is typically about 600 to 650° C. and the reaction pressure is typically about 40 to 80 kpa. The steam acts as a diluent to lower the partial pressure of the hydrogen by-product produced by the dehydrogenation reaction, allowing the reaction to proceed to a greater extent. The steam also provides the heat to drive the dehydrogenation reaction, which is highly endothermic, and decreases the amount of coke formation on the reactor catalyst by steam gasification. This process consumes high amounts of energy through the use of excess steam, and the energy required to vaporize and superheat the steam. It also has the disadvantages of catalyst deactivation and limited thermodynamic conversion.
The Lummus/UOP Smart Process is another process for conversion of EB to styrene that addresses some of the problems of direct dehydrogenation by using selective oxidation of a portion of the hydrogen by-product formed in the dehydrogenation reaction. The exothermic oxidation reaction of the hydrogen with oxygen provides at least part of the heat required for subsequent EB dehydrogenation. The removal of hydrogen from the process shifts the reaction equilibrium in the dehydrogenation unit to substantially increase single-pass EB conversions while maintaining high styrene monomer selectivity. Drawbacks of this process include the need for two catalysts in the reactor, one to catalyze the dehydrogenation reaction and a second catalyst for the oxidation of the hydrogen by oxygen. Reactor design and catalyst loading is more complicated in this system. Formation of aromatic oxidants in the reactor and CO2 production can adversely affect the potassium-promoted iron oxide dehydrogenation catalyst. Also, there are safety concerns when injecting oxygen into a hydrocarbon mixture.
More recently, the use of CO2 as a mild oxidant has been proposed. In a process described in U.S. Pat. No. 6,958,427, ethylbenzene is dehydrogenated to styrene monomer in the presence of carbon dioxide as a soft oxidant over a catalyst comprising vanadium and iron, with the CO2 being externally supplied from the discharge of another petrochemical process. Compared with the conventional process, the presence of carbon dioxide allows operation at a lower temperature and provides enhanced conversion and significant energy savings. The use of CO2 as an oxidant avoids the explosion risks of oxygen and shows high selectivity and conversion at lower temperatures than direct dehydrogenation. The CO2 may also function as a heating medium and replace some or all of the steam used in conventional dehydrogenation processes.
The problems associated with this process are well known and described in U.S. Pat. No. 6,958,427, the entire contents of which are incorporated herein by reference. For example, drawbacks include high investment and operating cost due to the following: 1) the need for an externally supplied source of CO2, such as the off-gas from an ethylene oxide plant; 2) the continued need for superheated steam as both a source of oxygen for “shifting” of by-product CO back to CO2, and a source of at least part of the heat required for the endothermic reaction of EB to SM; 3) the need for a water/gas shift reactor; and 4) the need for separation of hydrogen from the water/gas shift reactor effluent; and/or 5) the need for separation of CO2 from the dehydrogenation reactor off-gas, requiring an elaborate scrubbing/stripping operation; and 6) the need for a hydrogenation reactor (reverse water/gas shift reactor). The need for a continuous supply of CO2 also limits the possible locations of the SM plant, since it must be located nearby a dedicated supply of CO2. It is important to recognize that there is no net elimination of CO2 by this process, despite claims that this is a “green” process. CO2 is simply an oxygen carrier, which is converted to CO in the oxydehydrogenation reactor. The CO must be converted back to CO2 by the water/gas shift reactor, or used to form some other oxygenated compounds.
The Oxirane POSM process produces SM as a co-product beginning with the oxidation of ethylbenzene to form ethylbenzene hydroperoxide intermediate, and subsequent epoxidation of propylene with the ethylbenzene hydroperoxide to yield equi-molar amounts of propylene oxide and styrene monomer. This process is extremely capital intensive and its economics are driven by the propylene oxide market.
In addition to the processes described above, the oxidative dehydrogenation of EB using oxygen as the oxidant, the Snamprogetti/Dow SNOW™ process (concurrent dehydrogenation of ethane and ethylbenzene), the Exelus ExSyM™ process (based on toluene and methanol feedstocks), a liquid-phase ethylbenzene dehydrogenation process (Pincer catalyst technology), and processes using membranes have been considered. These processes have not been demonstrated commercially.
It would be desirable to have a process for production of styrene by dehydrogenation of EB that avoids one or more of the drawbacks of prior dehydrogenation processes.